The present invention relates to an ion source apparatus and, more particularly, to an ion source apparatus of electron-bombardment field-emission type in which an ion source material is heated to cause surface ionization thereby producing an ion beam through an applied electric field of a high strength.
An ion source of the aforementioned type may be used for fabrication of submicro-structures of LSIs (Large Scale Integrated Circuits), the measurements of the order of sub-microns in secondary ion mass spectrometry and others.
Surface ionization type ion sources have been proposed which include a resistance heater and an emitter tip coupled to the heater, with an ion source material being supplied to the coupled portion. The emitter tip may be either a sharp-pointed end member or be fashioned as a porous structure.
Disadvantages of the proposed prior art ion systems reside in the fact that such systems generally exhibit poor heating deficiency characteristics. To improve the heating efficiency, it is necessary to maintain the ion source material at a temperature higher than the melting point; however, such approach results in an evaporation of the ion source material resulting in a wastage or consumption of the source material as well as a contamination of the environment.
Additionally, in extreme cases, the evaporated ion source material may be deposited on a high voltage insulation resulting in an electrical breakdown and thereby adversely shortening the useful service life of the ion source and degrading the realiability of the ion source apparatus.
A further disadvantage of problem encountered in the prior art ion source apparatus resides in the fact that reaction of the ion source material with a heater material may occur, since electrically conductive materials, in general exhibit increased tendency for mutual reaction and are likely to be molten at relatively low temperatures. Further, a reaction with the material of the emitter tip may also take place. Consequently, not only is the sharp-pointed end of the emitter tip dulled, but also the wastage or dissipation of the tip material becomes significant. Under the circumstances, there is imposed restriction on the types of ions which are allowed to be extracted as the ion beam.
A further crucial problem common to the prior art ion sources which operate on the heat transfer principle resides in the fact that a selection of the emitter tip material from the electrically conductive materials is indispensably required notwithstanding the fact that the electrically conductive materials present the problem of the reaction mentioned above. Besides, the resistance heating provides an obstacle to the attempt for increasing the temperature of the emitter tip.
It is an object of the present invention to provide an ion source apparatus which avoids the disadvantages and difficulties encountered in prior art ion sources and which is capable of reducing the wastage of the ion source material and environmental contamination, increasing the number or types of ions to be produced, extending a service life of the ion source apparatus, and enhancing the reliability thereof.
In view of the above object, in accordance with the present invention an ion source apparatus is provided which includes a round rod-like emitter tip having a sharp-pointed end, with an ion source material holder for holding the emitter tip coaxially within a crucible made of a material having a high melting point. The crucible has an opening formed in the bottom wall thereof through which the sharp-pointed end of the emitter tip extends to the exterior, with an ion source material being filled around the outer periphery of the sharp-pointed end of the emitter tip. A filament emits an electron beam for bombarding the emitter tip with electrons from below, and a heating power supply is provided for the filament. An ion beam extracting electrode is disposed between the emitter tip and is the filament and maintained at a substantially same potential as the filament accelerating voltage power supply applies a high voltage between the beam extracting electrode and the emitter tip to accelerate the electrons and the ion beam.
By virtue of arrangement in accordance with the present invention, the sharp-pointed end of the emitter tip is heated directly by the electron bombardment or electron rays, whereby an improved heating efficiency can be achieved as compared with the prior art resistance heating. Furthermore, due to the heating through electron bombardment, a high temperature of 3000.degree. C. or more can be easily attained. Moreover, a structure in which the ion source material is held within the crucible at a lower portion thereof permits wastage or loss of the ion source material due to evaporation to be significantly reduced as compared with that of the prior art ion sources. Additionally, protection is provided against the environmental or ambient contamination or pollution due to atom vapor. Also, since, in accordance with the invention, an insulation material of a low thermal conductivity can be used as the material for the emitter tip, the heating efficiency can be enhanced while the reaction with the ion source material can be avoided. Furthermore, by virtue of the two superposed ionization mechanisms of the surface ionization and the electron bombardment, an ion beam of an increased intensity can be produced with an ion source constructed in accordance with the present invention.